1. Field of the Invention
The present invention relates to a transistor circuit, and more particularly, it relates to a transistor circuit such as a constant current source circuit, a voltage-to-current conversion circuit, a level shifter or the like.
2. Description of the Background Art
FIG. 15 is a circuit diagram showing a conventional constant current source circuit. In the circuit shown in FIG. 15, a resistor 31 is provided between an emitter of a bipolar transistor 30 and a ground 57, and a bias potential V.sub.B is supplied to a base of the transistor 30 to feed a current I to its collector. FIG. 16 shows a graph illustrating the state of change of a collector current IC which flows with respect to a collector-to-emitter voltage V.sub.CE of a bipolar transistor. There is the so-called pentode region PR, in which a substantially constant collector current IC flows without depending on the collector-to-emitter voltage V.sub.CE. It is necessary to drive the transistor in this pentode region PR, so that the circuit shown in FIG. 15 operates as a constant current source circuit. In other words, it is necessary to sufficiently increase output resistance.
Considering this with reference to FIG. 15, the potential at the emitter of the transistor 30 reaches a value which is lower than the bias potential V.sub.B by a base-to-emitter voltage V.sub.BE when the bias potential V.sub.B of 1.2 V, for example, is supplied to the base of the transistor 30. Since the voltage V.sub.BE is about 0.7 V in general, the emitter potential is about 0.5 V. Thus, a voltage of 0.5 V is applied to the resistor 31. Assuming that R represents the resistance value of the resistor 31, the following current flows to the collector: EQU I=(V.sub.B -V.sub.BE)/R (1)
and the circuit shown in FIG. 15 operates as a current source circuit which feeds a current of the value determined by equation (1). A collector potential of the transistor 30 must be in excess of the bias potential V.sub.B, so that the transistor 30 operates in the pentode region.
This also applies to a MOS transistor circuit shown in FIG. 17. In this circuit, which comprises a MOS transistor 32 whose source is connected to a ground 57, there is also such a pentode region PR as that shown in FIG. 16 regarding the collector current I.sub.C as a drain current I.sub.D and the collector-to-emitter voltage V.sub.CE as a drain-to-source voltage V.sub.DS in the figure.
Considering this with reference to FIG. 17, a bias potential V.sub.B of 2.0 V, for example, is supplied to the gate of a transistor 32. In this case, a current I flowing to the drain is determined by the following equation of the pentode region PR of the MOS transistor: EQU I=(.beta./2)(V.sub.GS -V.sub.TH).sup.2 ( 2)
where .beta. represents a constant called a transistor gain factor which is in proportion to the gate width of the transistor and in inverse proportion to the gate length, V.sub.GS represents a gate-to-source voltage, and V.sub.TH represents a threshold voltage, which is about 0.8 V in general. When the drain-to-source voltage V.sub.DS of the MOS transistor 32 satisfies the following relation: EQU V.sub.DS .gtoreq.V.sub.GS -V.sub.TH ( 3)
the MOS transistor 32 operates in the pentode region PR. Namely, the transistor circuit shown in FIG. 17 operates as a constant current source.
In current mirror circuits shown in FIGS. 18 and 19, it is also preferable that transistors forming the same operate in pentode regions.
While the conventional constant current source circuit has been structured in the aforementioned manner, its constant current property has yet been insufficient. A slight increase .DELTA. of the collector current I.sub.C (or the drain current I.sub.D) due to the increase of the collector-to-emitter voltage V.sub.CE (or the drain-to-source voltage V.sub.DS) shown in FIG. 16 is caused by an Early effect in the bipolar transistor (or a channel length modulation effect in the MOS transistor).
In order to reduce this, circuits of FIGS. 20 and 21 which are obtained by increasing single transistors in respective ones of the circuits shown in FIGS. 15 and 17 are also employed. In the circuit shown in FIG. 20, a collector current I.sub.C is commonly fed in bipolar transistors 33 and 34, and an emitter of the transistor 34 is connected to a ground 57 through a resistor 31, similarly to the circuit shown in FIG. 15. However, it is necessary to set two bias potentials V.sub.B1 and V.sub.B2 in this case and hence a plurality of bias circuits are required.
In the circuit shown in FIG. 21, a drain current I.sub.D is commonly fed in MOS transistors 35 and 36, and a source of the transistor 36 is connected to a ground 57 similarly to the circuit shown in FIG. 17. Also in this case, two bias potentials V.sub.B1 and V.sub.B2 must be set similarly to the circuit of FIG. 20.
This also applies to such a case that a plurality of constant current sources are connected in parallel. FIG. 22 is a circuit diagram showing a constant current circuit which is formed by bipolar transistors 33a, 33b, . . . 33n and 34a, 34b, . . . , 34n and resistors 31a, 31b, . . . 31n. This circuit has such a structure that circuits shown in FIG. 20 are connected in parallel. Namely, bases of the transistors 33a, 33b, . . . , 33n and bases of the transistors 34a, 34b, . . . , 34n are connected in common respectively, while the resistors 31a, 31b, . . . , 31n, which are connected to respective emitters of the transistors 34a, 34b, . . . , 34n are connected in common at a ground 57. Also in this circuit, it is necessary to set two potentials, i.e., a bias potential V.sub.B1 to be supplied to the bases of the transistors 33a, 33b, . . . , 33n and a bias potential V.sub.B2 to be supplied to the bases of the transistors 34a, 34b, . . . , 34n.
A circuit shown in FIG. 23 has such a structure that circuits shown in FIG. 21 are connected in parallel. Bases of MOS transistors 33a, 33b, . . . , 33n and bases of MOS transistors 36a, 36b, . . . , 36n are connected in common respectively, and it is necessary to set two bias potentials V.sub.B1 and V.sub.B2.
An insufficient constant current property of a transistor causes a problem also in another transistor circuit. FIG. 24 is a circuit diagram showing an emitter follower which is formed by a bipolar transistor 49 and a current source 50. An input terminal 52 and an output terminal 51 are connected to a base and an emitter of the transistor 49 respectively, to cause level shift by a base-to-emitter voltage V.sub.BE of the bipolar transistor 49. However, since the amount of level shift is varied with an input potential which is received in the input terminal 52 due to an Early effect of the bipolar transistor 49, an output potential obtained from the output terminal 51 is disadvantageously distorted. This also applies to a source follower circuit shown in FIG. 25. Although the amount of level shift is provided by V.sub.TH +.sqroot.(2I/.beta.) and can be adjusted by the value I of a current which is fed by a constant current source 56 in this circuit, an output potential outputted from an output terminal 55 which is connected to an emitter in common with the constant current source 56 is disadvantageously distorted with respect to an input potential received in an input terminal 53 which is connected to a base of a MOS transistor 54 since a constant current property is insufficient due to a channel length modulation effect of the MOS transistor 54.
In the emitter follower circuit shown in FIG. 24, further, the amount of level shift is substantially fixed only at V.sub.BE. While it is possible to insert a diode 58 between an emitter of a transistor 49 and a current source 50 as shown in FIG. 26 in order to further increase the amount of level shift, the amount of level shift substantially reaches 2V.sub.BE in this case and this circuit cannot be used to obtain an amount of level shift between V.sub.BE and 2V.sub.BE. While it is alternatively possible to adjust the amount of level shift by employing a resistor in place of the diode, such a passive element occupies a large area and unpreferably hinders integration and speeding up.
On the other hand, such a problem that two potentials must be set is also caused in a voltage-to-current conversion circuit. FIG. 27 shows an exemplary structure thereof. Bipolar transistors 37 and 38 form a differential pair, while a bipolar transistor 39, a resistor 40 and a ground 57 are identical in structure to the constant current source shown in FIG. 15. When an input potential V.sub.in is supplied to a base of the transistor 37, complementary collector currents flow to terminals 41 and 42 in response to difference between the input potential V.sub.in and a bias potential V.sub.B2. Also in this circuit, therefore, it is necessary to set two potentials, i.e., a bias potential V.sub.B1 for the constant current source and a bias potential V.sub.B2 forming the reference of voltage-to-current conversion.
This also applies to an ECL type inverter shown in FIG. 28. Bipolar transistors 45 and 46 form a differential pair, while a bipolar transistor 47, a resistor 48 and a ground 57 are identical in structure to the constant current source shown in FIG. 15. Single load resistors 44 are connected to respective collectors of the transistors 45 and 46. When an input potential V.sub.in is supplied to a base of the transistor 45, a large current flows to a transistor which comprises a base receiving a higher one of the input potential V.sub.in and a bias potential V.sub.B2, and a logic taken out from a terminal 43 inverts a logic expressed by the input potential V.sub.in. Also in this circuit, it is necessary to set two bias potentials V.sub.B1 and V.sub.B2.
The aforementioned problems are summarized as follows:
(1) A constant current source circuit including one transistor may have an insufficient constant current property, and when the constant current source circuit is formed by employing two transistors which are fed with a common output current in order to solve this, two bias voltages must be set and hence a plurality of bias circuits are required.
(2) A plurality of bias circuits are also required in a constant current source circuit, such as a current mirror circuit, for example, in which a plurality of constant current source circuits of the item (1) are connected in parallel with each other.
(3) Also in a conventional voltage-to-current conversion circuit, it is necessary to set two voltages of a bias voltage forming the reference of an input voltage and a bias voltage which is necessary for a portion serving as a constant current source. This also applies to an inverter.
(4) In a conventional level shift circuit, an output potential is disadvantageously distorted since a drain current has an insufficient constant current property. Further, the amount of level shift can only be set at a level of about integral times of V.sub.BE.